The present invention relates generally to optical fiber sensors. More particularly, the present invention relates to a micromachined optical fiber pressure sensor.
Optical fiber sensors are used to measure temperature, pressure, displacement and the like. Optical fiber sensors have several advantages over electronic sensors. Significantly, optical fiber sensors are not affected by electromagnetic noise. Also, optical fiber sensors can have higher temperature capability than electronic sensors.
Presently there is a demand for an optical fiber pressure sensor capable of measuring pressure in the combustion chamber of an internal combustion engine. Electronic sensors do not function well in an internal combustion engine because of high temperatures and electromagnetic noise created by the engine. An optical fiber pressure sensor capable of operation in an internal combustion engine would allow for dynamic fuel efficiency control and other benefits.
The prior art describes optical fiber pressure sensors having a Fabry-Perot cavity at the end of an optical fiber. Variation in applied pressure alters the Etalon cavity length and hence the optical response of the Etalon cavity. A sensor system coupled to the optical fiber measures applied pressure by monitoring the optical characteristics of the Etalon.
Some of the prior art pressure sensors have disadvantages, including:
1) a nonlinear response to temperature changes. This is a problem where the pressure sensor must operate linearly over a wide range of temperatures (e.g. in an internal combustion engine);
2) absence of a mechanism for providing an accurate fiber-diaphragm spacing (i.e. etalon cavity length). This is particularly important in devices where the fiber endface functions as a reflector in an etalon. In such devices the optical fiber must be accurately longitudinally located with respect to the diaphragm; and
3) too many parts. Several of the prior art devices require precise alignment of small parts, which is expensive and difficult.
It would be an advance in the art of optical pressure sensors to provide a pressure sensor that is simple to assemble with an accurate etalon cavity length, requires a small number of parts and can tolerate high heat.
The present invention is directed to an optical fiber pressure sensor. The apparatus provides a linear pressure response, is simple in construction and passively provides an accurate etalon cavity length.
The present apparatus comprises a base layer with an optical fiber hole for receiving an optical fiber. A fiber stop layer is disposed on the base layer over the optical fiber hole. A diaphragm cap layer is disposed over the fiber stop layer. The diaphragm cap layer has a diaphragm aligned with the optical fiber hole in one embodiment of the invention.
In use, an optical fiber is typically disposed in the optical fiber hole in order for the device to operate as intended.
The optical fiber is butted against the fiber stop layer in one embodiment of the invention. The fiber stop layer may or may not have a hole aligned with the optical fiber hole. Butting the optical fiber against the fiber stop layer provides an accurate etalon cavity length.
The apparatus may include an etch stop layer disposed between the fiber stop layer and base layer. The fiber can be butted against this as well. The etch stop layer may or may not have a hole aligned with the optical fiber hole.
In an aspect of the invention, the etch stop layer has a thickness in the range of about 0.04 to 2 microns. Optionally, the etch stop layer has a thickness less than 1/250 the thickness of the base layer, or less than 1/50 the thickness of the fiber stop layer. The base layer can have a thickness in the range of about 125-1000 microns; the fiber stop layer can have a thickness in the range of about 10-100 microns.
The etalon cavity length can be in the range of 20-200 microns.
The diaphragm can be spaced apart from the fiber stop layer, or bonded directly to the fiber stop layer. The diaphragm cap layer can have an etched pit, or the diaphragm cap layer can be a flat layer.
Optionally, the base layer and diaphragm cap layer are made of the same material. This tends to reduce possible problems associated with thermal expansion mismatches. The fiber stop layer can be made of a material different from the base layer and diaphragm cap layer.
Optionally, the base layer, fiber stop layer and diaphragm layer are made of single crystal silicon. Also optionally, the etch stop layer is made of SiO2, glass, alumina or silicon nitride.